Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and improve fuel economy. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As one example, an intake pressure, such as throttle inlet pressure (TIP), may be used to adjust an output of an intake oxygen sensor (IAO2) in order to compensate for pressure fluctuations in the air intake system that may affect the IAO2 output. This compensation may be possible due to a direct correlation between the time-varying signals of the TIP sensor and IAO2 sensor. However, the inventors herein have recognized potential issues with this method. As one example, IAO2 sensor degradation resulting in a slow responding IAO2 sensor (e.g., an output having an altered time constant) may degrade the correlation between the TIP and IAO2 signals, thereby resulting in an inaccurately corrected IAO2 output. More specifically, as a time constant of the IAO2 signal varies relative to a time constant of the TIP signal, pressure correction of the IAO2 signal based on the TIP signal may have reduced accuracy. As a result, EGR estimates based on the corrected IAO2 signal may have reduced accuracy, thereby degrading EGR control and engine control based on EGR flow estimates.
In one example, the issues described above may be addressed by a method for indicating degradation of an intake oxygen sensor based on a first time constant of an output of the intake oxygen sensor and a second time constant of an output of a throttle inlet pressure sensor. In this way, a vehicle operator may be notified when replacement and/or repair of the oxygen sensor is necessary. Additionally, EGR control based on inaccurate EGR flow estimates from a degraded oxygen sensor may be avoided by using another method to determine EGR flow estimates when the intake oxygen sensor has become degraded.
For example, the oxygen sensor may be positioned in an intake passage of the engine, downstream of where an EGR passage couples to the intake passage. As such, the intake oxygen sensor may be used to estimate EGR flow based on an output of the intake oxygen sensor relative to a reference output determined when EGR was not flowing to the intake passage. A TIP sensor may be positioned in the intake passage downstream of and/or proximate to the oxygen sensor, but upstream of an intake throttle. As such, the TIP sensor may be used to estimate the TIP. During engine conditions such as when the engine is boosted, the TIP and oxygen concentration may fluctuate over the same time intervals. When the TIP fluctuates by more than a threshold amount, the amount of time it takes the TIP sensor to register the change in TIP may be estimated. A similar estimate may be made for the time it takes the oxygen sensor to register the change in oxygen concentration. Based on the delays of the two sensors, degradation of the oxygen sensor may be detected if the delay in the oxygen sensor is different than the delay of the TIP sensor by more than a threshold amount. By detecting oxygen sensor degradation, a vehicle operator may be notified when replacement of the oxygen sensor is necessary, and EGR estimates using the oxygen sensor may be avoided when the oxygen sensor is degraded.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.